Cytokinins are a well-known class of plant growth hormones active in promoting cell division, cell growth and differentiation, and other physiological processes. In particular, cytokinins are active in processes regulating disease resistance, stress tolerance, drought tolerance, resistance to lodging, delayed senescence, apical dominance, and assimilate partitioning in a plant, Werner et al., Proc. Natl. Acad. Sci, 98(18)10487–10492 (2001), Haberer et al., Plant Physiol., 128, pp.354–362 (2002).
Senescence, which constitutes the final phase of development in plants, is a critical stage of the plant life cycle. It is part of the aging process that typically occurs before cell death and is characterized by changes in cell structure, metabolism and gene expression that effect a decline in the activities of plants. Inhibiting senescence in a plant has been identified as a way to prolong the active life-span of a plant. Certain hormones associated with senescence, e.g., cytokinin, when present in increased levels in plants, have been demonstrated to delay senescence and prolong plant activity.
It has been previously demonstrated that plants with altered senescence patterns have leaves that retain high levels of chlorophyll throughout seed and flower development. Tobacco plants with altered leaf senescence patterns have enhanced yield of biomass and flower, see U.S. Pat. No. 5,689,042.
Because of the importance of plants for food production, there is a continuous and substantial effort to improve plants, e.g., create plants with increased disease resistance phenotypes, increased stress and drought tolerant phenotypes, increased resistance to lodging phenotypes, delayed senescence phenotypes, apical dominance phenotypes, and assimilate partitioning phenotypes. Plants with improved phenotypes are better able to meet the demands of food production. Accordingly, there is a need to create plants with improved phenotypes. This invention addresses this and other needs.